1. Technical Field
The present invention falls within the aeronautical industry and relates to aircraft stringers. More particularly it relates to “T” shaped composite stringers and methods of making these.
2. Background Art
In aircraft construction, a stringer or longeron or stiffener is a thin strip made of carbon fiber. Several strips are co-bonded on a cured skin in order to build a part of the aircraft (e.g. wings, HTP, etc.).
A very common type of stringers is that with a “T” shaped cross section defining a stringer web and a stringer foot.
Normally, a method of manufacturing a “T” shaped stringer for an aircraft comprises a first step of hot-forming laminates in order to achieve semi-stringers with an “L” shaped cross section, and a second step of placing together two hot-formed semi-stringers to achieve a “T” shaped stringer. Then, the “T” shaped stringer is co-bonded on a cured skin with an adhesive line between them.
It is also well known in the aeronautical industry that such method comprises a third step, being a curing cycle, in which curing tools, preferably invar alloy angles of approximately 90°, are placed following the contour of the hot-formed “T” shaped stringer inside a vacuum bag during the curing cycle. It should be noted that invar angles are of 90° exactly when the skin on which the stringers are co-bonded is a plane skin. But, when the skin is curved, then the angle between the foot and the web of a stringer vary slightly, thus making the invar angle not exactly 90°.
These “T” shaped stringers, being formed under the traditional manufacturing methods in which uncured stringers are co-bonded on cured skins with the stringer curing tool inside the vacuum bag, are subjected to a “Mexican hat effect” drawback which occurs when the stringer thickness becomes:                smaller than the nominal thickness at the end of the stringer web, and        greater than the nominal thickness at the ends of the stringer feet.        
Thus, if any other piece has to be assembled on the stringer feet and the difference between the thicker and the thinner points of the stringer feet is too great, then a solid shim has to be added in order to level the surface on which the other piece has to be assembled. The use of these solid shims increases assembly difficulties and the amount of work involved.
ES Patent Application P201030219 describes a traditional method of manufacturing stringers especially with a “T” shaped cross section.
It was, therefore, desirable to find a method of manufacturing stringers in which the “Mexican hat effect” drawback explained above is minimized.